1. Field of the Invention
This invention relates generally to medical devices, such as stents and covered stents. More particularly, the present invention is directed to a structure on the surface of the stent and a method for forming the structure.
2. Description of the Background
Certain implantable medical devices, such as stents and grafts, are implanted within blood vessels and other body passageways to treat disease conditions such as stenoses, occlusions, and aneurysms.
Stents are scaffoldings, usually cylindrical in shape that function to physically support, and, if desired, expand the wall of the passageway. Typically, a stent consists of two or more struts or wire support members connected together into a lattice-like or open weave frame.
Most stents are compressible for insertion through small cavities, and are delivered to the desired implantation site percutaneously via a catheter or similar transluminal device. Once at the treatment site, the compressed stent is expanded to fit within or expand the lumen of the passageway. Stents are typically either self-expanding or are expanded by inflating a balloon that is positioned inside the compressed stent at the end of the catheter. Intravascular stents are often deployed after coronary angioplasty procedures to reduce complications, such as the collapse of arterial lining, associated with the procedure.
In addition to providing physical support to passageways, stents are also used to carry therapeutic substances for local delivery of the substances to the damaged vasculature. For example, anticoagulants, antiplatelets, and cytostatic agents are substances commonly delivered from stents and are used to prevent thrombosis of the coronary lumen, to inhibit development of restenosis, and to reduce post-angioplasty proliferation of the vascular tissue, respectively. The therapeutic substances are typically either impregnated into the stent or carried in a polymer that coats the stent. The therapeutic substances are released from the stent or polymer once it has been implanted in the vessel.
A problem with delivering therapeutic substances from a stent is that, because of the limited size of the stent, the total amount of therapeutic substance that can be carried by the stent is limited. Furthermore, when the stent is implanted into a blood vessel, much of the released therapeutic substance enters the blood stream before it can benefit the damaged tissue. To improve the effectiveness of the therapeutic substances, it is desirable to maximize the amount of therapeutic substance that enters the local vascular tissue and minimize the amount that is swept away in the bloodstream.
The lattice-like structure of the stent leaves spaces defined by the struts that form the stent. These spaces can allow plaque from the lesion to fall through the stent and enter the blood stream during stent deployment. The spaces can also permit malignant tissue growth through the stent openings into the body passageway and can allow undesired contact between blood flowing through the blood vessel and damaged portions of the vessel intima. Covered stents, in which a polymeric material surrounds and is attached to the stent, have been proposed to alleviate the problems associated with stent openings.
Diseased vessels are also treated with grafts. Grafts are generally tubular in shape and are used to replace or create an anatomical passageway to provide a new conduit for fluid, e.g. blood. Grafts are often made from a portion of a vein, but can also be constructed from a synthetic material to form a synthetic graft. Like stents, synthetic grafts can be positioned percutaneously via a catheter, for instance, to be placed at the site of an aneurysm to prevent further dilation and possible rupture of the diseased vessel.
In certain instances, the graft material alone does not provide enough structural support for the graft, causing the graft to collapse and occlude or impede the flow of blood through the vessel. To counter this problem, a similar, even identical, structure to the covered stent, in which a stent is placed within the synthetic graft material, has been proposed to improve the structural strength of grafts. This structure is sometimes referred to as a synthetic stent graft. Stents are also placed at the ends of synthetic grafts to help secure the ends of the synthetic graft to vessel walls.
Examples in the patent literature of covered stents include U.S. Pat. No. 5,948,191 titled "Low profile, thermally set wrapped cover for a percutaneously deployed stent" issued to Solovay; U.S. Pat. No. 5,123,917 titled "Expandable intraluminal vascular graft" issued to Lee; U.S. Pat. No. 5,948,018 titled "Expandable supportive endoluminal grafts" issued to Dereume et al.; U.S. Pat. No. 5,282,824 titled "Percutaneous stent assembly" issued to Gianturco; U.S. Pat. No. 5,843,164 titled "Intraluminal stent for attaching a graft" issued to Franzen; and U.S. Pat. No. 6,010,530 titled "Self-expanding endoluminal prosthesis" issued to Goicoechea.
A problem with covered stents and synthetic stent grafts is keeping the stent covering attached to the stent. During expansion of the prosthesis, the covering pulls isometrically, causing the cover to shorten and possibly detach from the stent.
Currently, covers are attached to stents by stitching or gluing, or by wholly embedding the stent into the polymeric cover material. When stitches are used, the cover is typically punctured at the stitch site, leaving an opening and a weak place in the cover that may tear or rip when the covered stent is expanded. Using glue instead of stitches eliminates these problems, however, glue can be difficult to keep in place on the stent when attaching the cover material. Furthermore, in some cases, the glue itself does not provide a strong enough hold to keep the cover attached. When the stent is wholly embedded into the cover material, the covering is on both the inside and outside of the stent and may cause the profile of the covered stent to be larger than desired.